I created this rifle using jack streat's mechanism, and improvising where neccessary.
It has a 15 brick magazine, with working reflex and iron sights.
It is accurate up until about 20 feet, then it starts to spin out of control.
I also modified the cocking handle so that it looks relatively like an M16 cocking handle.
The instructions below are jack streat's instructions, I am just re-posting them. If you want to build the entire rifle, just go to mocpages and type in "HK416 CQB".
Here are some picture of the rifle from all angles and aspects.